As a method for increasing a thermal efficiency of an engine, the technique of providing a heat-insulating layer on a surface facing a combustion chamber of an engine was proposed in the 1980s (see, for example, Patent Document 1). Since then, a heat-insulating layer of a ceramic sintered body and a heat-insulating layer of a flame-sprayed layer containing ZrO2 particles showing low thermal conductivity have been proposed.
In application as a heat-insulating layer, however, the ceramic sintered body was faced with the problem of occurrence of cracks and peeling in the heat-insulating layer due to thermal stress or thermal impact. Thus, the ceramic sintered body has not been put into practical use especially for a heat-insulating layer in a relatively large portion such as a top face of a piston, an inner peripheral surface of a cylinder liner, and a lower face of a cylinder head.
On the other hand, the flame-sprayed layer itself once was used for a cylinder liner and a trochoid surface of a rotary engine. This use, however, was intended to increase the abrasion resistance, and was not intended to increase heat insulation. To use the flame-sprayed layer as a heat-insulating layer, a low-thermal-conduction material containing zirconia (ZrO2) as a main component is preferably flame sprayed. However, adhesion among particles in a zirconia-based layer is inferior to that of a cermet-based layer, and cracks easily occur due to, for example, thermal stress and fatigue caused by repetitive mechanical stress.
On the other hand, Patent Document 2 proposes that a heat-insulating thin film composed of a particulate first heat insulator and a film-shape second heat insulator is provided in a member facing a combustion chamber of an engine. Patent Document 2 shows that the second heat insulator has the function of bonding the first heat insulator, ceramic such as zirconia (ZrO2), silicon, titanium, or zirconium, ceramic containing carbon and oxygen as main components, or ceramic fibers having high strength and high thermal resistance is used as a second heat insulator, and the second heat insulator is applied as a coating or bonded onto a base material.
Patent Document 3 shows a technique for forming a heat-insulating thin film on a member facing an engine combustion chamber. In this technique, a thin film is formed on a combustion chamber wall surface of the member by applying a mixture of an organic silicon compound and large number of resin grains and heating the thin film so as to form a large number of bubbles in the member. Specifically, the thin film is heated to a temperature of 600 to 800° C. or more, thereby gasifying the resin grains. In addition, a gas generated by thermal decomposition of the organic composition is purged from the thin film, and the thin film after the gas purge is heated to a higher temperature (1000 to 1200° C. or more). In this manner, a silicon compound (SiO2 and SiC) generated by the thermal decomposition of the organic silicon compound is sintered.